1. Field of the Invention
This invention relates to a method for control of a gallium diffusion with at least one silicon wafer in the open tube of a diffusion oven, whereby a gas mixture of water vapor and mainly hydrogen is passed over gallium oxide (Ga.sub.2 O.sub.3) and the volatile sub-oxide (Ga.sub.2 O) produced by reduction is then carried in the gas stream to the silicon wafers, optionally with additional control of the gallium concentration in the silicon by dilution of the gas mixture with nitrogen.
2. Description of the Prior Art
In a known such open gallium diffusion (Frosch, Derick: J. Electrochem. Soc. 105, p. 695, 1958) there is used for control of the gallium concentration in the silicon a water bath through which hydrogen, and optionally nitrogen, is passed in order to humidify the gas (Becker, Fischer, Popp: BMFT-Forsch-ungsbericht T 76-16, pp. 27-51, July 1976). The humidified gas is directed into a diffusion oven. The water bath is held at different temperatures according to the gallium concentration sought. For example, an increase in the boundary concentration is obtained by lowering the bath temperature. Additionally, the doping concentration can be adjusted by diluting the gas mixture with nitrogen.
If in the known case it is desired to vary the gallium concentration in the course of the diffusion as is always necessary when complicated diffusion profiles (double profiles) are to be produced, then, as already mentioned, the bath temperature must be varied. This can happen only with the time delay necessary for reaching a new temperature equilibrium. The known method is consequently subject to the following disadvantages:
(1) A slow adjustment of the concentration due to the thermal inertia of the water baths;
(2) The process cannot be made automatic;
(3) The baths must be attended, and a long-duration process, e.g. over the weekend, is not possible since the baths must be refilled at intervals of about 24 hours on account of evaporation;
(4) Baths through which the gases must be passed represent a source of contamination for the gases.
Impurities of the water bath reaching the diffusion tube with the water vapor can form recombination centers in the silicon and thereby reduce the carrier lifetime.
Thus, it is desirable to employ other techniques for producing the requisite mixture of water vapor and hydrogen for gallium diffusion.
Systems are known (company brochure of the Tylan Corporation "Gas Systems for H.sub.2 -O.sub.2 Oxidations with HCR" R-6193-2 "A" of April 1972, pp. 1-7; company brochure of the Lindberg Co. (USA) "Burnt Hydrogen Oxidation Systems", Specification 87042, p. 112) in which hydrogen and oxygen are burnt in an oxidation oven. The water vapor is accordingly produced directly in the oxidation oven as the result of the chemical combustion reaction. In the known "Lindberg" case the application to diffusion processes is also mentioned. These known systems allow for the oxidation process a better control of the gas composition; less contamination of the gases; the possibility of automatic control through synchronized operation of the valves in the supply lines for hydrogen, oxygen and nitrogen, as well as easier manipulation, since no water baths must be attended. Thus far, however, it has found no application to gallium diffusion. Rather, it is operated with the stoichiometric mixture of 1:2 required for complete combustion of oxygen and hydrogen, or for safety reasons--even with an excess of oxygen so as to guarantee that under all circumstances no free hydrogen is left over after the reaction. Further, in the known case the nitrogen serves merely as a cleansing gas and is directed through the hydrogen nozzle. The oxygen line must, in the known case, be connected to the HCl line necessary there, in order to be able to carry out a final cleansing phase to prevent corrosion.